Rats are implanted with chronic stimulating electrodes in various midbrain sites wherein stimulation with minute amounts of electrical current is aversive. The rats are then trained to bar press to escape the aversive stimulation in the decremental bar-pressing paradigm. In this paradigm, each bar press by the animal decrements the administered current by 5 percent of the initial level. Various drugs which are known to modify the biochemistry of brain biogenic amines are administered to the animals, and subsequent changes in bar-pressing are noted. The ultimate goal of this research is to obtain insights into the neurobiology of aversive mental states in man.